A proposal exists for a horizontal navigation system for an Automated Ground Vehicle (AGV) including a carrier wave difference Global Positioning System (GPS) device and a laser scanner (see for example Japanese Patent Application Laid-Open (JP-A) No. 2006-138834). In the system of JP-A No. 2006-138834, the navigation error is estimated using a tightly-coupled Extended Kalman Filter (EKF). A double difference code and carrier wave phase from a two frequency GPS receiver and relative position arising from laser scanner measurements are used as EKF measurement values.
In positioning employing GPS data, when a single frequency GPS receiver is used there is an issue that since the location is estimated based on a state amount of the current time data and immediately preceding time data, the solution readily falls into a local solution and is susceptible to noise. According to the system of JP-A No. 2006-138834, improvements can be obtained in measurement precision using a frequency diversity effect. However, two frequency GPS receivers are much more expensive than single frequency GPS receivers, and hence suffer from a lack of general applicability.